For the past 20 years, the integrated circuit (IC) device density has doubled about every 18 months. When the gate length of integrated circuits is less than 0.18 .mu.m, the propagation time or delay time is dominated by interconnect delay instead of device gate delay. To address this problem, new materials with low dielectric constants are being developed. The aim of this development is to decrease time constant (RC delay), decrease power consumption, and decrease cross-talk in integrated circuits. There are two groups of low K dielectric materials, the traditional inorganic group exemplified by SiO.sub.2, and newer organic polymers, exemplified by poly(para-xylylene). Organic polymers are considered an improvement over inorganic low dielectric materials because the K of organic polmers can be as low as 2.0. However, most of the currently available organic polymers have serious problems. Specifically, they have insufficient thermal stability, and are difficult and expensive to manufacture in a vacuum system.
For IC features of 0.35 .mu.m, current production lines use materials consisting primarily of SiO.sub.2. The SiO.sub.2 products have dielectric constants ranging from 4.0 to 4.5. In addition, stable fluorinated SiO.sub.2 materials with a dielectric constant of 3.5 have been achieved. These F--SiO.sub.2 -containing materials are primarily obtained from plasma enhanced chemical vapor deposition (PECVD). and high density, plasma chemical vapor deposition (HDPCVD) of various siloxane containing compounds such as trimethylsiloxane (TMS), tetraethylorthosilicate (TEOS) and silazanes in conjunction with SiF.sub.4, C.sub.2 F.sub.4.
1. Precursors and Polymers
Several thermally stable polymers or polymer precursors arc under study. These include polyimides (PIM), fluorinated polyimides (F-PIM). polyquinoxalines (PQXL), benzocyclobutenes (BCB), fluorinated polyphenylethers (F-PPE), and several types of silsesquisiloxanes. These polymers have dielectric constants ranging from 2.6 to 3.0. Solutions of these polymers or their precursors are used in spin coating processes to achieve gap filling and planarization over metal features. However, the dielectric constants of these polymers is too high for the future ICs faith small feature sizes. In addition, all thermally stable polymers including PIM and PQXL have a persistent chain length (PCL; or the loop length of a naturally curling up polymer chain) up to several hundred or thousands of .ANG.. Long PCL makes complete gap filling very difficult if not physically impossible.
Recently, another type of low dielectric material, poly(para-xylylene) (PPX) has been studied and evaluated for future IC fabrication. These PPX include Parylene-N.TM., Parylene-C.TM. & Parylene-D.TM. (trademarks of Special Coating System Inc.'s poly(para-xylylenes). Currently, all commercially available poly(para-xylylenes) are prepared from dimers. The currently available starting materials or dimers for manufacturing poly(para-xylytenes) arc expensive (&gt;$500 to $700/kg). Unfortunately, these poly(para-xylylenes) have high dielectric constants (K=2.7-3.5) and low thermal stability (decomposition temperature, Td is &lt;320.degree. C.-350.degree. C. in vacuum), and thus are not suitable for IC fabrication requiring high temperature processing.
The fluorinated poly(para-xylylene) (F-PPX) or Parylene AF-4.TM., for example, has the structure of (--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --).sub.n. It has a dielectric constant of 2.34 and is thermally stable (0.8%/hr. wt. loss at 450.degree. C. over 3 hours in nitrogen atmosphere).
II. Processes for Manufacturing Polymers
Currently fluorinated poly (para-xylylenes) are polymerized from F-dimers by the method of Gorham, (J. Polymer Sci. A1(4):3027 (1966)) as depicted in Reaction 1 below: ##STR1##
In this reaction Ar is --C.sub.6 H.sub.4 --. However, the precursor molecule and the F-dimer needed for the manufacture of Parylene AF-4.TM. is expensive and time-consuming to make because several chemical reaction steps are needed to make its fluorinated dimer.
Fluorinated dimers are manufactured according to the following series of chemical steps: ##STR2##
The overall yields for making F-dimers is low (estimated from 12% to 20% based on the weight of its starting material). In addition, the last step of the syntheses of the precursor. or the dimerization step (4a or 4b), can only be effectively carried out in very dilute solutions (from 2% to less than 10% weight/volume) resulting in low conversion efficiecy. Further, the needed lead time and material cost for making F-containing dimers is very high. For instance, 10 g of the F-dimer can cost as much as $2,000/g. The lead time is 2-3 months for getting 1 kg of sample from current pilot plant production facilities.
Therefore, even though fluorinated poly(para-xylylenes) might be suitable as dielectric materials in "embedded" IC structures, it is very unlikely that the F-dimer will ever be produced in large enough quantity for cost effective applications in future IC fabrication.
On the other hand, a readily available di-aldehyde starting material (Compound Ia) is reacted with sulfurtetrafluoride at an elevated pressure of 1 MPa to 20 MPa and temperatures of 140.degree. C. to 200.degree. C. to yield the tetrafluorinated precursor (Compound IIIa) and sulfur dioxide (Reaction 2). The sulfur dioxide is then exhausted from the reaction chamber. Alternatively, the di-aldehyde can be reacted with diethylaminosulfur trifluoride (DAST) at 25.degree. C. at atmospheric pressure to make the Compound IIIa. ##STR3##
Y is H, and Ar is phenylene moiety. Both Compound Ia and Compound IIIa have a non-fluorinated phenylene moiety. The Compound IIIa in solution can be converted into a dibromo Compound IIIb (see below, Reaction 3) through a photo-reaction (Hasek et al., J. Am. Chem. Soc. 82:543 (1960). The dibromo Compound IIIb (1-5%) was used in conjunction with CF.sub.3 --C.sub.6 H.sub.4 --CF.sub.3 by You, et al., U.S. Pat. No. 5,268,202 to generate di-radicals (Compound IV) that was transported under low pressure to a deposition chamber to make thin films of fluorinated poly(para-xylylenes). ##STR4##
Additionally, poly(para-xylylene)-N (Parylene-N.TM. or PPX-N) was also prepared directly from pyrolysis of p-xylene. (Errede and Szarwe, Quarterly Rev. Chem. Soc. 12:301 (1958); Reaction 4). According to this publication, highly cross-linked PPX-N was obtained. ##STR5##
III. Deposition of Polymer Films
The deposition of low dielectric materials onto wafer surfaces has been performed using spin on glass (SOG), but for newer devices which have features smaller than 0.25 .mu.m, SOG processes cannot fill the small gaps between features. Therefore, vapor deposition methods are preferred. Of these, transport polymerization (TP) and chemical vapor deposition (CVD) are most suitable.
In both TP and CVD, the precursor molecule is split (cracked) to yield a reactive radical intermediate which upon deposition onto the wafer can bind with other reactive intermediate molecules to form a polymer. The polymer thus forms a thin film of material with a low dielectric constant.
Chemical vapor deposition has been used to deposit thin films with low dielectric constant. Sharangpani and Singh, Proc. 3d Int. DUMIC Conference. 117-120 (1997) reported deposition of amorphous poly(tetrafluoroethylene) (PFTE; Teflon.TM., a registered trade name of DuPont, Inc.) by a direct liquid injection system. A solution of PFTE is sprayed on a wafer substrate, which is exposed to ultraviolet light or light from tungsten halogen lamps. Unfortunately, PFTE has a low glass transition temperature (Tg) and cannot be used for IC fabrication requiring temperatures of greater than 400.degree. C.
Labelle et al., Proc. 3d Int. DUMIC Conference, 98-105 (1997) reported using pulsed radio frequency (RF) plasma enhanced CVD (PECVD) process for deposition of hexafluoropropylene oxide. However, as with poly(tetrafluoroethylene), the resulting polymers have low Tg values and cannot be used as dielectrics.
Kudo et al., Proc. 3d Int. DUMIC Conference, 85-92 (1997) reported using a PECVD process for deposition of hydrocarbons including C.sub.2 H.sub.2 /(C.sub.2 H.sub.2 +C.sub.4 F.sub.4).
Lang et al., Mat. Res. Soc. Symp. Proc. 38 1:45-50 (1995) reported thermal CVD process for deposition of poly(naphthalene) and poly(fluorinated Naphthalene). Although polymers made from these materials have low dielectric constants, the polymers are very rigid, being composed of adjoining naphthalene moieties. Thus, they are prone to shattering with subsequent processing such as Chemical Mechanical Planarization (CMP).
Selbrede and Zucker, Proc. 3d Int. DUMIC Conference, 121-124 (1997) reported using a thermal TP process for deposition of Parylene-N.TM.. The dielectric constant of the resulting polymer (K=2.65-2.70) also was not low enough. For future IC applications, the decomposition temperature (Td) of the thin film was also too low to withstand temperatures greater than 400.degree. C.
Wang et al., Proc. 3d Int. DUMIC Conference, 125-128 (1997) reported that annealing a deposited layer of poly(para-xylylene) increases the thermal stability, but even then, the loss of polymer was too great to be useful for future IC manufacturing.
Wary et al. (Semiconductor International, June 1996. pp: 211-216) used a fluorinated dimer. the cyclo-precursor (.alpha., .alpha.,.alpha.',.alpha.', tetrafluoro-di-p-xylylene) and a thermal TP process for making polmers of the structural formula: {--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --}.sub.n. Films made from Parylene AF-4.TM. have dielectric constant of 2.28 and have increased thermal stability compared to the hydrocarbon dielectric materials mentioned above. Under nitrogen atmosphere, a polymer made of Parylene AF-4.TM. lost only 0.8 % of its weight over 3 hours at 450.degree. C.
In contrast to a CVD process, transport polymerization (TP) (Lee, C. J., Transport Polymerization of Gaseous Intermediates and Polymer Crystal Growth. J Macromol. Sci. -Rev. Macromol. Chem. C16:79-127 (1977-1978), avoids several problems by cracking the precursor in one chamber and then transporting the intermediate molecules into a different deposition chamber. By doing this, the wafer can be kept cool, so that deposited materials on the wafer are not disrupted, and multiple layers of interconnect devices may be manufactured on the same wafer. Further, the conditions of cracking can be adjusted to maximize the cracking of the precursor, ensuring that very little or no precursor is transported to the deposition chamber. Moreover, the density of the transported intermediates may be kept low, to discourage re-dimerization of intermediates. Thus, the thin film of low dielectric material is more homogeneous and more highly polymerized than films deposited by CVD. These films have higher mechanical strength and can be processed with greater precision, leading to more reproducible deposition and more reproducible manufacturing of integrated circuits.
Among all currently available poly(para-xylylenes), F-PPX (--CF.sub.2 --C.sub.6 H.sub.4 --CF.sub.2 --).sub.n or Parylene AF-4.TM. has the lowest dielectric constant and best thermal stability. This resulted from a lower polarity and higher bonding energy of C--F bond compared to those of C--H bond. So far, the F-PPX is considered to be the most promising "embedded" IMD for future 0.18 .mu.m ICs due to its low dielectric constant (K=2.34) and high thermal stability (0.8%/hr. wt. loss at 450.degree. C. up to 3 hours). However, to be useful as an interlevel dielectric material, a lower K (K&lt;2.3-2.5) polymer still needs to have better thermal stability, T.sub.d and thermal mechanical strength than those of the Parylene AF-4.TM., as higher T.sub.d, glass transition temperature T.sub.g and Elastic Modulus are needed for re-flow or annealing of aluminum or copper. In addition, higher Tg and Elastic Modulus (E) are desirable for CMP to achieve global planarization. In this invention, new chemical compositions are provided to overcome the above mentioned problems.